Recirculation of a controlled amount of diesel engine exhaust gas to the engine air intake is generally known to provide a beneficial reduction in diesel engine emissions of oxides of nitrogen oxide. Limits are conventionally imposed on the amount of recirculated exhaust gas to avoid excessive engine intake air charge dilution which may degrade engine performance and increase particulate emission levels.
An internal combustion engine may operate in a variety of different closed loop and open loop modes based upon a variety of monitored engine operating parameters. Some typical operating closed loop modes include a cold mode, a warm mode, a cranking mode, a low idle mode, and a high idle mode. In-between the low idle mode and the high idle mode, the engine may run in an open loop mode where fuel is delivered based on engine throttle position and engine speed. Various engine operating parameters may be monitored to determine the engine operating mode including engine speed, throttle position, vehicle speed, coolant temperature, and oil temperature, as well as others.
Various control systems for exhaust gas recirculation are known in the art. U.S. Pat. No. 5,377,651 discloses a closed-loop control system for exhaust gas recirculation wherein the calculations for determining the amount of exhaust gas to recirculate are computed continuously throughout the engine's operational cycle. However, it may be undesirable to have the exhaust gas recirculated during certain engine operating modes, such as when the engine is cold or cranking, or when it is undesirable to reduce the amount of air available for combustion. It is also desirable to stop recirculating exhaust gas if the engine has been idling for an extended period of time. The calculations for determining the amount of exhaust gas to recirculate in the engine consume valuable data processing time. It is therefore desirable to include means in a system to bypass the calculations for the amount of exhaust gas to recirculate, thereby disabling exhaust gas recirculation, when the engine is operating in a mode where there is no need for recirculated exhaust gas.
Accordingly, the present invention is directed to overcoming one or more of the problems as set forth above.